The invention relates to an internal combustion engine comprising a cylinder block with cooling fins cast integral therewith, and a cylinder head bolted on top of the cylinder block, first coolant passages being provided in the area of at least one cylinder contained in the cylinder block, which passages are connected to second coolant passages located in the cylinder head.
Air-cooled internal combustion engines offer a number of advantages when used with motorcycles, carts, etc. Simplicity of design is one of them, as no coolant pumps, external radiators or the like will be required. Besides, outward appearance of the engine often is important since an engine rigged up with cooling fins is regarded as aesthetically pleasing. The drawback of air-cooled engines is their lesser performance as a consequence of the limited possibility of heat dissipation, and non-uniform heat distribution due to the fact that the internal heat exchange will not be as efficient as with liquid-cooled engines. In motorcycles, for example, the front sides of the cylinders are usually cooled much more efficiently than the back sides on account of the ambient airflow prevailing. In the instance of V-engines where the crankshaft is positioned transversely to the longitudinal axis of the vehicle, thermal conditions at the rear cylinder usually are less satisfactory.
In WO 84/01979 a cooling system for internal combustion engines is disclosed where the cooling medium evaporates partially. Condensation takes place in an external radiator. The design of such a cooling system is relatively complex.
It is an object of this invention to propose an internal combustion engine where the above disadvantages are eliminated while the advantages of an air cooling system as described above will be maintained and engine performance may be increased by improving cooling efficiency.
According to the invention this object is achieved by providing heat exchange passages in the outer region of the cooling fins, which together with the first and second coolant passages form a closed loop system within the engine. By means of this simple design a liquid cooling system may be obtained which will not necessitate an external radiator. Thus the outward appearance of motorcycles and other vehicles will not be impaired by such a radiator. The invention is essentially based on the fact that, while the thermal efficiency of cooling fins will increase with their height, any additional increase beyond a certain limit will improve cooling only marginally. Thanks to the invention it will also be possible to efficiently utilize the outer region of the cooling fins for the cooling system.
An especially favorable design solution is achieved by providing connecting bores in the lower part of the cylinder block, in order to connect the heat exchange passages to the first coolant passages. Considering that the heat exchange passages run essentially vertically when the engine is mounted in the vehicle, the thermosiphon effect, which causes the coolant to circulate, will be optimized. Most advantageously, the coolant will be allowed to reach boiling temperature in the first coolant passages when the engine is subjected to extremely high loads. Boiling of the coolant will accelerate its circulation considerably and thus improve cooling efficiency. To ensure that pressures will remain within acceptable limits in the boiling state of the coolant, it may be provided by the invention that an expansion tank be formed in the upper part of the cylinder head, which is designed for volume compensation.
In a particularly favorable variant of the invention the heat exchange passages are located in the outer region of the cooling fins. In this way the cooling fins may be utilized as heat exchange surfaces against the ambient air to special advantage.
An especially favored design is obtained by combining a number of heat exchange passages into a cooling bank which is connected to the other coolant passages via a single opening. In this manner the number of connections requiring seals between the cylinder block and the cylinder head may be minimized.
An internal combustion engine of great thermal efficiency will be obtained by surrounding the engine cylinder by several cooling banks. Preferably, the cooling bank is connected to the first coolant passages via a single connecting bore. In this way an excellent internal heat exchange will be possible.
No coolant pump will be required if the first coolant passages, and the second coolant passages, and the heat exchange passages form a closed loop system within which the coolant is allowed to circulate freely.
Due to the absence of mechanically moveable parts a long-lived system of great robustness will be obtained.